1. Field of the Invention
The present invention relates to a semiconductor photodetecting device and a method for fabricating the semiconductor photodetecting device, more specifically a semiconductor photodetecting device having an optical waveguide for leading light to a photodetector formed integral therewith and a method for fabricating the semiconductor photodetecting device.
2. Description of the Related Art
With the recent development of information communication networks represented by internets, faster optical communication systems are increasingly required. This also requires semiconductor photodetecting devices used in the detection of optical signals, etc. in the optical communication systems to be operative fast at information transfer speeds of above 40 Gbit/s.
As a semiconductor photodetecting device which is operative fast, the semiconductor photodetecting device invented by the inventors of the present application, which comprises a tapered optical waveguide which can be easily connected to optical fibers, and a photodiode, which are integral with each other is known (see, e.g., Japanese Patent Laid-Open Publication No. 2002-26370).
FIGS. 19A-19D are diagrammatic views of the structure of the prior art semiconductor photodetecting device integrally including the tapered optical waveguide. FIG. 19A is a sectional view of the prior art semiconductor photodetecting device in the direction of light propagation. FIG. 19B is the sectional view along the line A-A′ in FIG. 19A. FIG. 19C is the sectional view along the line B-B′ in FIG. 19A. FIG. 19D is the sectional view along the line C-C′ in FIG. 19A.
An optical waveguide unit 102 for incident light to propagate through, and a photodetection unit 104 for detecting the light which has propagated through the optical waveguide unit 102 are disposed adjacent to each other on an SI(Semi-Insulating)-InP substrate 100.
An n type InP layer 106 is formed on the SI-InP substrate 100.
A tapered InGaAsP core layer 108 having the thickness continuously increased from the end of the SI-InP substrate 100 toward the photodetection unit 104 is formed on the n type InP layer 106 of the optical waveguide unit 102. An InP clad layer 110 is formed on the n type InP layer 106 with the InGaAsP core layer 108 buried in.
A non-doped InGaAs photoabsorption layer 112 is formed on the n type InP layer 106 of the photodetection unit 104. A p type semiconductor layer 114 is formed on the InGaAs photoabsorption layer 112. Thus, a PIN photodiode 116 having the InGaAs photoabsorption layer 112 sandwiched by the n type semiconductor layer 114 and the n type InP layer 106 is formed.
The end surface of the PIN photodiode 116 on the side of the optical waveguide 102 is optically coupled to the end surface of the InGaAsP core layer 108 on the side of the photodetection unit 104.
A p type electrode 118 is formed on the p type semiconductor layer 114 of the PIN photodiode 116. An n type electrode 120 is formed on the n type InP layer 106 of the photodetection unit 104.
Light which has be led to the semiconductor photodetecting device by an external optical waveguide, such as an optical fiber or others, is incident on one end of the InGaAsP core layer 108 at the end of the buried optical waveguide unit 102.
The light incident on the InGaAsP core layer 108 propagates through the InGaAsP core layer 108 toward the photodetection unit 104 to be incident on the side surface of the InGaAs photoabsorption layer 112 of the PIN photodiode 116.
The PIN photodiode 116 on which the light which has propagated through the InGaAsP core layer 108 is incident outputs electric signals corresponding to intensities of the incident light to the p type electrode 118, based on the principle of photoelectric conversion.
The prior art semiconductor photodetecting device shown in FIGS. 19A-19D can have response characteristics of 40 GHz, and the inventors of the present application, et al. have fabricated test semiconductor photodetecting devices having no dependence on polarized wave, and having higher photodetecting efficiencies (see, e.g., CPT2001 Technical Digest (2001) p.105).
For the above-described prior art semiconductor photodetecting device having the tapered optical waveguide to realized high speed operation, the capacitance of the photodetection unit must be small. For the low capacitance of the element, it will be most effective to form the element in mesa. The PIN photodiode is formed in mesa, whereby the PIN junction capacitance can be halved in comparison with that of the prior art structure.
However, it has been difficult in the fabrication to monolithically integrate the mesa-shaped photodetection element and the tapered optical waveguide. This is because it is difficult that the mesa-shaped photodetection element and the tapered optical waveguide are concurrently patterned and etched, and accordingly the optical coupling loss between the tapered optical waveguide and the mesa-shaped photodetection element becomes large.